Allergic disorder

In this CCC, we will cover a few conditions such as contact dermatitis, allergic rhinitis, insect bites and stings and, food allergy and anaphylaxis.

CONTACT DERMATITIS

Contact dermatitis is either an allergic or irritant skin reaction. We can classify them as either of the 2 types:

⚠️ Risk factors

• Occupational exposures - labourers, food-industry workers, machine operators, farmers, healthcare professionals, janitors, dry cleaners, cooks, florists, beauticians and hairdressers.
• Atopic dermatitis (eczema) - increased risk of developing ICD (but not ACD).

😷 Presentation

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Irritant contact dermatitis:

Presents within minutes-hours of exposure to severe irritants. Mild irritants can take days-weeks to present with ICD.

It most commonly occurs on hands and face and is limited to the site of exposure:

• Erythema
• Burning
• Pustules or acneiform lesions
• Ulceration - may occur with severe irritants.

😷

Allergic contact dermatitis:

ACD presents within 24-72 hours of exposure (previously sensitised).

It often affects the dominant hand but anywhere may be affected. Often it can affect the margins of the hairline.

• Erythema
• Pruritus
• Vesicles and bullae
• Uriticaria - with exposure to latex and certain foods.

🔍 Investigations

1. Patch testing - allergens are presented onto the skin using a path and this will show inflammation which can be graded on a scale within 2-7 days of application if positive for a specific allergen.
2. Repeated open application test (ROAT)/provocative use test (PUT) - the test is performed 2x daily for 1 week and best simulates the contact dermatitis caused by leave-on products (moisturisers, sunscreens and cosmetics).

😷 Management

Of course for both ICD and ACD, the first-line option is to avoid irritants/allergens.

• Emolients should be applied to the affected area in ICD.
• Topical corticosteroids - such as hydrocortisone can be used for ACD.

INSECT BITES AND STINGS/VENOM ALLERGY

Let’s start by discussing insect bites and insect stings:

• Insect bites

Insects such as mosquitoes, bed bugs and fleas all have piercing mouthparts that can pierce the skin. Other insects such as horseflies lacerate the skin instead.

Insects often contain antigenic components in their saliva that can cause local, or systemic reactions. Usually the first time they are bitten there is no reaction (unless directly toxic). However, a type 4 hypersensitivity reaction develops and after repeated bites it can lead to inflamed and itchy maculopapules.

More concerningly, a type 1 hypersensitivity reaction may develop with a typical wheal and flare presentation. This develops within 20 minutes of the bite, and can sometimes lead to anaphylaxis.

• Insect stings

Bees, wasps and hornets inject venom from a sac attached to a stinger into the skin directly. This venom contains allergens along with histamine to cause reactions that can be either classified as local or systemic

1. Local - these are no more than 10cm from the site of exposure. Redness, swelling and pain are limited to the soft tissue solely.
2. Systemic - these are distant from the site, such as widespread redness, itching, uriticaria, angioedema. It is more likely to lead to anaphylaxis if there is airways and haemodynamic compromise.

😷 Presentation

Local symptoms

• Swelling
• Erythema
• Pain

Systemic symptoms

• Uriticaria
• Rhinitis
• Wheezing
• Abdominal pain
• Vomiting
• Dizziness
• Angioedema
• Anaphylaxis

🔍 Investigations

Investigations are not usually required as a clinical diagnosis can be made based on symptoms and history.

⚠️ We need to test for venom allergies in any patient with a history of systemic reaction or anaphylaxis (skin prick or venom-specific IgE testing) and if they are positive, venom immunotherapy (VIT) should be provided.

🧰 Management

• If the stinger is visible → remove it.

• Small local reaction → no management other than symptomatic management.
• Pain - paracetamol or ibuprofen to ease the pain.
• Itching - chlorphenamine (antihistamine) or hydrocortisone cream may help too.
• Large local reaction → often oral antihistamines and/or oral corticosteroids may be given (but be careful of prescribing corticosteroids if there is a risk of the bite/sting being infected).

• Systemic reaction → admission to A&E is required and may need treatment for anaphylaxis in the interim. An allergy specialist referral should be made before discharging the patient.

FOOD ALLERGY

Food allergy refers to an adverse immune response to proteins found within food. Common food allergies include peanuts, tree nuts, milk, eggs, fish, shellfish, wheat and soya.

It is likely to occur from genetic and environmental factors both and can be IgE mediated (acute) and non-IgE mediated (subacute/chronic) it may also have .

😷 Presentation

🔍 Investigations

1. Skin prick - is generally first-line. Skin prick is more reproducible, faster (15 minutes) and is cheaper to perform with a good sensitivity (only 50% specificity however). Positive allergens produce a wheal diameter >3mm compared to control is positive.
2. Serum specific IgeE immunoassay - also can be used but is more expensive and takes longer to produce results.
3. Food challenges - these are ultimately the best tests but they are long, expensive and difficult to perform as staff and equipment on hand to treat anaphylaxis is necessary.

🧰 Management

Anaphylaxis needs to be managed accordingly (see notes on anaphylaxis).

Avoidance and allergy action plan should be put in place when patients are stable.